1. Field of the Invention
This invention relates to the alignment and switching of nematic liquid crystal devices.
2. Discussion of Prior Art
Liquid crystal (LC) devices typically comprise of a thin layer of a liquid crystal material contained between cell walls. Optically transparent electrode structures on the walls allow an electric field to be applied across the layer causing a re-ordering of the liquid crystal molecules.
There are three known types of liquid crystal material nematic, cholesteric and smectic each having different molecular ordering. The present invention concerns devices using nematic materials.
In order to provide displays with a large number of addressable elements it is common to make the electrodes as a series of row electrodes on one wall and a series of column electrodes on the other cell wall. These form e.g. an x,y matrix of addressable elements or pixels and for twisted nematic types of device are commonly addressed using rms addressing methods.
Twisted nematic (TN) and phase change devices are switched to an ON state by application of a suitable voltage and allowed to switch to an OFF state when the applied voltage falls below a lower voltage level, i.e. these devices are monostable. For a twisted nematic type of device (90xc2x0 or 270xc2x0 twist as in U.S. Pat. No. 4,596,446) the number of elements that can be rms addressed is limited by the steepness of a device transmission verses voltage curve (as described by Alt and Pleschko in IEEE Trans Ed vol ED 21, (1974) P.146-155). One way of improving the number of pixels is to incorporate thin film transistors adjacent to each pixel: such displays are termed active matrix displays. An advantage of nematic types of devices is the relatively low voltage requirements. They are also mechanically stable and have a wide temperature operating range. This allows construction of small and portable battery powered displays.
The main disadvantages of the above devices are as follows. The 90xc2x0 twisted nematic has a poor viewing angle characteristic which leads to loss of contrast when the device is viewed at high incident angles in certain azimuthal directions. Furthermore greyscale inversion occurs in these orientations. The low steepness of the 90xc2x0 twisted nematic can be improved by increasing the twist angle to 180xc2x0-270xc2x0. However this generally leads to no improvement in viewing angle characteristic. Both types of device also suffer from the fact that the large difference in the nematic tilt between the on and off states leads to a change in pixel capacitance which can cause crosstalk problems with other pixels.
The above disadvantages can be overcome by using a voltage controlled twist configuration which was first described in GB96/07854 and PCT/GB97/01019. This device uses a surface which can induce a voltage dependent twisting torque. One example of the VCT configuration is shown in FIG. 3. In this example the configurations at an applied voltage of zero or V1 will both appear dark when the cell is placed between crossed polarisers oriented along the x and y axes. At V2 the twist in the cell leads to guiding of the light and so this state will be bright between crossed polarisers. In order to achieve to achieve high contrast across a wide viewing angle, the dark state (at V1) should have a very low transmission regardless of viewing orientation. The optimum liquid crystal configuration to obtain this property is a planar non twisted structure in which the tilt angle is low throughout the thickness of the cell. The structure at V1 is closer to this optimum than that shown at zero volts. Unfortunately a fully planar non twisted state is only obtained if the groove depth of the grating is small as this will delay the onset of twist to much higher voltages. However low voltage operation is preferred especially for active matrix applications to allow low power operation and compatability with low cost electronics. In order to achieve low voltage operation with the VCT, the twist threshold must occur at low voltage which means that significant splay will exist in the dark state. In the worst case the system will switch directly from a hybrid state (V=0 in FIG. 3) to the twisted state at V2.
According to this invention the viewing of a non-optimum VCT can be improved by adding one or more birefringent layers disposed on one side or on both sides of the cell. Birefringent layers have been used (EP-0686869, EP-0676660, EP-0622656) for symmetric liquid crystal configurations, eg twisted nematic devices. However, the VCT is a hybrid configuration in which conventional arrangements of compensating layers would not be expected to be effective.
According to this invention an improved VCT liquid crystal device comprises:
a nematic or chiral nematic liquid crystal material of negative dielectric anisotropy;
two containing cell walls, spaced apart by spacers and carrying electrodes or other means to impose a field on the liquid crystal layer;
an aligning surface treatment at one cell wall providing a substantially planar alignment of the liquid crystal with a defined azimuthal alignment direction;
an aligning surface treatment on the second cell wall providing a preferred, substantially homeotropic alignment of the adjacent liquid crystal material, and also provides a defined azimuthal alignment direction to the adjacent liquid crystal, when said liquid crystal near the second cell wall is caused to depart significantly from the said preferred, substantially homeotropic alignment.
means to cause optical modulation of light passing through the device;
CHARACTERISED BY
one or more birefringent compensator layers disposed adjacent one or on both cell walls in order to improve the viewing characteristics of the display.
The retardation of the compensator layers may have the same, greater, or smaller retardation than that of the liquid crystal material.
The birefringent compensator layers may be films of polymer rendered birefringent, for example, by a controlled stretching process (Y Fujimura, et al SID 92 Digest p.397 (1992)). Alternatively the birefringent layers may comprise a further cell filled with a liquid crystal composition together with the necessary surface treatments to impose a defined state of alignment. Optionally a field may be applied to such a cell to further control its state of alignment. Alternatively the birefringent layer may comprise an aligned liquid crystal polymer film (e.g. S. T. WU, SID Applications Digest p.21 (1996)). Other known materials and their combinations such as quartz or calcite slices, or oblique evaporants (e.g. J. P. Eblem et al SID 97 Digest p.683 (1977)), may be also used.
The electrodes may be formed as a series of row and column electrodes arranged and an x,y matrix of addressable elements or display pixels. Typically, the electrodes are 200 mm wide spaced 20 mm apart.
Alternatively, the electrodes may be arranged in other display formats e.g. r-xcex8 matrix or 7 or 8 bar displays.